Instead of golf club shafts made of steel, golf club shafts made of FRP (Fiber Reinforced Plastics) which are formed by winding prepregs, made of reinforced fibers (e.g., carbon fibers) impregnated with a thermosetting resin, into layers and thermally curing the same have been widely used.
FIG. 6 is a schematic perspective view illustrating a configuration example of a typical conventional golf club shaft 1. The golf club shaft 1 includes a compressive rigidity (crush rigidity) holding layer 2, a torsional rigidity holding layer 3 and a bending rigidity holding layer 4, in that order toward the outer layer side from the inner layer side. The compressive rigidity holding layer 2 is configured of a prepreg (90-degree (hoop) layer prepreg) whose fiber direction is orthogonal to the longitudinal direction of the shaft, the torsional rigidity holding layer 3 is configured of a prepreg (biasing prepreg; prepreg of a 45-degree layer) whose fiber direction is inclined to the longitudinal direction of the shaft, and the bending rigidity holding layer 4 is configured of a prepreg (prepreg of a 0-degree layer) whose fiber direction is parallel to the longitudinal direction of the shaft. The compressive rigidity holding layer 2 is sometimes layered on top of the torsional rigidity holding layer 3. The prepregs configuring the compressive rigidity holding layer 2 and the bending rigidity holding layer 4 are usually referred to as UD (unidirectional) prepregs since the fibers thereof extend in a single direction. Furthermore, the torsional rigidity holding layer 3 usually includes a pair of UD prepregs (45-degree layers/biasing prepregs) whose fiber directions are symmetrical with respect to the longitudinal direction of the shaft (generally ±45° relative to the longitudinal direction); in addition, the applicant of the present invention has also developed the golf club shaft 1 in which a plain weave fabric (biaxial woven fabric) prepreg, a triaxial woven fabric prepreg and a tetra-axial woven fabric prepreg that are made by impregnating a plain weave fabric (biaxial woven fabric), a triaxial woven fabric and a tetra-axial woven fabric with a thermosetting resin, respectively, are incorporated in the torsional rigidity holding layer 3.
On the other hand, the trend in weight reduction of golf club shafts of recent years has been significant, and the applicant of the present invention has been promoting the development of ultra-lightweight golf club shafts having a total weight of 35 grams or less.
A simple manner of reducing the weight of a golf club shaft is to reduce the number of prepregs constituting the golf club shaft and to reduce the total weight by reducing the thickness and density of each prepreg. However, when it is attempted to achieve reduction in weight of a golf club shaft to the limit, e.g., a reduction to 35 grams or less, this attempt is always accompanied by the problem of reduction in the strength (especially bending strength and torsional strength) of the golf club shaft, thus increasing the possibility of the gold club shaft being damaged. In other words, in the technical field of golf club shafts, there is basically a trade-off relationship between the shaft weight reduction and shaft strength maintenance, and conventional golf club shafts are still incapable of meeting the demand for both shaft weight reduction and shaft strength maintenance.